Magnetoresistive Random Access Memory (MRAM), based on the integration of silicon CMOS with MTJ technology, is a major emerging technology that is highly competitive with existing semiconductor memories such as SRAM, DRAM, and Flash. Spin-transfer torque (STT) magnetization switching described by C. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), has stimulated considerable interest due to its potential application for spintronic devices such as STT-MRAM on a gigabit scale.
STT-MRAM has a MTJ element that is usually based on a tunneling magneto-resistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin non-magnetic dielectric layer. The MTJ element is typically formed between a bottom electrode such as a first conductive line and a top electrode which is a second conductive line at locations where the top electrode crosses over the bottom electrode. A MTJ stack of layers may have a bottom spin valve configuration in which a seed layer, reference layer, a thin tunnel barrier layer, a ferromagnetic “free” layer, and a capping layer are sequentially formed on a bottom electrode. The reference layer has a fixed magnetization direction. The free layer has a magnetic moment that is either parallel or anti-parallel to the magnetic moment in the reference layer. The tunnel barrier layer is thin enough that a current through it can be established by quantum mechanical tunneling of conduction electrons. When a sense current is passed from the top electrode to the bottom electrode in a direction perpendicular to the MTJ layers, a lower resistance is detected when the magnetization directions of the free and reference layers are in a parallel state (“0” or P memory state) and a higher resistance is noted when they are in an anti-parallel state (“1” or AP memory state). In STT-MRAM, the resistance can be switched between the two states by the application of a current pulse of sufficient magnitude to write the bit to the opposite state.
Compared with conventional MRAM, spin-transfer torque or STT-MRAM has an advantage in avoiding the half select problem and writing disturbance between adjacent cells. The spin-transfer effect arises from the spin dependent electron transport properties of ferromagnetic-spacer-ferromagnetic multilayers. When a spin-polarized current transverses a magnetic multilayer in a current perpendicular-to-plane (CPP) configuration, the spin angular moment of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and non-magnetic spacer. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high, and if the dimensions of the multilayer are small. STT-MRAM has the same read mechanism as MRAM but differs in the write operation mechanism.
Typically, the magnetic moments of the reference layer and free layer are in an in-plane direction. However, for a variety of reasons, it is advantageous to engineer perpendicular magnetic anisotropy (PMA) into the aforementioned layers so that their magnetization direction is perpendicular-to-plane. The source of PMA may be intrinsic or it may be induced in a ferromagnetic layer at interfaces with an oxide layer, for example, in situations where the ferromagnetic layer has a thickness less than a threshold level. Spintronic devices with perpendicular magnetic anisotropy have an advantage over MRAM devices based on in-plane anisotropy in that they can satisfy the thermal stability requirement but also have no limit of cell aspect ratio. As a result, spin valve structures based on PMA are capable of scaling for higher packing density which is one of the key challenges for future MRAM applications and other spintronic devices.
In a MTJ within a MRAM or STT-MRAM, a reference layer will typically exert a stray magnetic field upon the free layer that tends to favor either the P or AP state. The stray field (Ho) has a form similar to a non-uniform electric “fringing” field at the edges of a parallel plate capacitor. As depicted in FIG. 1, the stray field Ho 4 from reference layer 1 impinges on the free layer 3. Note that a dielectric spacer 2 such as a tunnel barrier layer separates the free layer and reference layer. When the reference layer 1 is a composite, the net stray field 4 will be the sum of fringing fields from several similar layers in the reference layer stack with the possible addition of a uniform effective “interlayer” coupling field. The free layer is subject to random thermal agitation and the stray field Ho will create a disparity in the thermal stability of the two states, with either the P or AP state rendered more thermally stable. This asymmetry is undesirable since for a given free layer coercivity (Hc), Ho should be zero for optimum stability.
Referring to FIG. 2, a synthetic antiferromagnetic (SAF) structure 18 is commonly employed as a reference layer to reduce the magnitude of Ho that impinges on a free layer 17. The SAF stack consists of two ferromagnetic layers labeled AP2 11, and AP1 13 which are coupled antiferromagnetically through an intervening non-magnetic layer 12 that is typically Ru. There is a tunnel barrier layer 16 formed between the SAF structure 18 and free layer 17. The net stray magnetic field Ho exerted by SAF structure 18 in a STT-MRAM bit with a 40 nm diameter is usually more than 500 Oe which is an unacceptably high value of about the same magnitude as the free layer Hc. Therefore, an improved reference layer is needed that generates a substantially smaller Ho than in current MTJ elements but has high thermal stability to at least 400° C., and maintains other magnetic properties such as a high magnetoresistance (MR) ratio.